(1) Field of the Invention
This invention relates to a method and apparatus for making snap acting thermally responsive bimetallic actuators for use in actuating the switch contacts of thermally responsive electric switches such as cut-outs, circuit breakers and thermostats.
(2) Description of the Prior Art
One well known form of bimetallic actuator is a bimetallic disc of domed configuration which moves to an oppositely domed configuration with a snap action with changes in temperature. Such discs are simple and inexpensive to mass produce, the basic operation of forming the actuator being pressing a flat disc between a pair of complementary steel dies to a desired domed configuration. Some form of heat treatment may be provided such as subjecting the actuator to a high temperature to stabilise the operating characteristics of the actuator, particularly the operating temperature.
For a variety of reasons, such discs have not proved stable over many repeated operations nor can they consistently be mass produced to an accurately predetermined operating temperature, i.e. the yield in such mass production of discs having a particular operating temperature is low, typically 40-50% for discs with an operating temperature within .+-.21/2.degree. C. of the desired operating temperature.
Another method of mass producing discs has been proposed employing fluid pressure instead of dies to deform a disc against a fixed stop, see British Patent Specifications Nos. 1027688, 1027689. The problem with such method, apart from practical difficulties in mass producing discs with such method, is that it is difficult accurately to impress a desired spherical deformation on the disc since the stop is not an accurate spherical abutment, which may cause kinking, and the relatively low fluid pressure against the natural resilience in the disc may result in a tendency of the disc to return a slight amount to its original shape when fluid pressure is released.
In copending U.S. application Ser. No. 639,856, filed Dec. 11, 1975 by the present applicant there is broadly disclosed and claimed a snap-acting thermally responsive bimetallic actuator comprising a member of sheet bimetal having an aperture with an arcuate outer perimeter and an inner perimeter defining a tongue free at one end, said free end being close to said outer perimeter, said outer and inner perimeters smoothly merging at rounded ends of the aperture adjacent the tongue root, an area of said member surrounding said tongue and in relation to which said tongue, at least in part, is generally centrally disposed having been deformed in a die pressing operation to conform in shape to a die of domed configuration, said domed area being such as to reverse its curvature with a snap action with change in temperature, and the width measured generally radially from the centre of said domed area of that part of the domed area in the region of the tongue root is substantially greater than the radial width of the part of said area generally opposite the free end of said tongue. In a preferred form such an actuator is domed overall and such preferred form of actuator will hereinafter be referred to as "an actuator as defined herein".
With the actuator as defined herein it has been found that a very much improved length of working life, stability of operating characteristics and accuracy in determining operating temperature is possible. Whilst such actuators could be mass produced by the methods outlined above it was preferred to develop a simple and inexpensive method of manufacture suitable for mass production which could realise to a greater extent the advantages inherent in such actuator.